The present invention relates to liquid crystal devices, and more particularly to a liquid crystal device comprising in combination a switching element and a bistable ferroelectric liquid crystal, as well as a method of driving the same.
Liquid crystal devices are used in a display, an optical shutter, a polariscope, etc. Especially, an active matrix display is a display wherein a display portion is formed with a switching element such as thin-film transistor (TFT), which is overlaid with a substance having electrooptic characteristics such as liquid crystal. It is a system suited to a display for high-definition and large-area use having a large number of scanning lines. In case of forming an active matrix wherein a liquid crystal, such as TN liquid crystal or guest/host liquid crystal, the light transmissivity of which depends upon the effective value of an applied voltage, and a TFT are stacked, a display whose number of scanning lines is large can be formed when the off-resistance R.sub.off of the TFT is sufficiently high and the on-resistance R.sub.on thereof is sufficiently low and when a time constant based on the current leakage of the liquid crystal itself is sufficiently great. However, a display of favorable characteristics cannot be formed if any of the parameters mentioned above is unsatisfactory.
The drive characteristics of an active matrix display employing a liquid crystal such as TN liquid crystal, the display state of which depends upon the effective value of an applied voltage, have been published by the inventors in Collection of Preliminary Manuscripts of All-Japan Meeting of the Television Society in 1983, pp. 121-122.
In view of the results, when the waveform of a voltage to be applied to a liquid crystal layer is calculated with a parameter being the off-resistance R.sub.off of a switching element such as thin-film transistor (TFT), a condition which permits display is V.sub.s &gt;V.sub.ns where V.sub.s denotes the effective value of a select voltage, while V.sub.ns denotes the effective value of a non-select voltage. It is understood that better display characteristics are attained as the off-resistance of the switching element is higher. In case of employing the TN liquid crystal in the active matrix display, therefore, improvements have been made in the direction of rendering the off-resistance R.sub.off of the switching element sufficiently high.
However, there has been known, for example, the phenomenon that the off-resistance of the TFT element lowers under the action of the light of a light source for illuminating the display. For coping with this, it has been proposed to hold the off-resistance R.sub.off high by a method for an amorphous silicon TFT, which stacks a metal film for intercepting light to enter a channel portion; a method for a polycrystalline silicon TFT, which changes an element structure; etc. Under the present situation, however, the TFTs cannot be said satisfactory for the characteristics of the switching elements for the active matrices, for such reasons that processes for manufacturing the elements become complicated and that the lowering of the off-resistance R.sub.off is inevitable as to intense light.
Ferroelectric liquid crystals having memory capabilities in themselves have been published in Applied Physics Letter, 36(11), pp. 899-901 or Molecular Crystal & Liquid Crystal, Vol 94, pp. 213-234 by N. A. Clark et al.; Japanese patent application laid-open No. 56-107216; etc. However, studies on the applications to actual displays cannot be said sufficient.